The present invention relates generally to implantable medical devices, and more particularly to a cardiac pacemaker which has a capability to be upgraded or modified non-invasively while surgically implanted, through remote programming.
Artificial cardiac pacemakers are typically implanted to control or correct an abnormality of the natural pacing or conduction system of a patient's heart. In general, the implanted pacemaker delivers an electrical pulse of selected magnitude to a predetermined location in the right atrium or ventricle to stimulate and pace the heart at a desired rate. Cardiac pacemakers for treating bradycardia and various other arrhythmias have evolved from the most simple asynchronous type in which the heart is paced at a fixed rate without sensing and thus without regard to the rhythmic relationship between the pumping chambers or the particular physiological requirements of the patient; to the synchronous type in which an event within the electrical activity of the heart is sensed and, depending on demand, a pulse is delivered (triggered) or not delivered (inhibited) at a fixed or base rate which takes into account the synchrony between the atrium and the ventricle; and finally, to the rate-adaptive or rate-responsive type that senses a physiological event which is indicative of the hemodynamic or metabolic demand of the patient, and delivers pacing pulses at a rate based on that demand as indicated by the nature and extent of the patient's physical activity (engaged in exercise) or lack thereof (merely at rest). An anti-tachycardia pacemaker is selected where the patient is suffering from episodes of an abnormally rapid irregular heart rate, and serves to break or terminate the tachycardia by delivering pulses to the appropriate chamber, typically the right ventricle, which are precisely timed relative to cardiac activity.
Various pacemaker codes have been devised, the original three letter ICHD code being perhaps most notable, to classify pacemakers according to function. The first letter is indicative of the chamber paced (atrium: A, ventricle: V, or both atrium and ventricle: D (for dual or double)); the second, the chamber sensed (A, V, D, or O--the latter indicating no sensing); and the third, the mode of response (triggered: T, inhibited: I, dual: D (for atrial triggered, ventricular inhibited), or none: O). Thus, for example, the outmoded DOO pacemaker was an atrioventricular (AV) synchronous sequential device in which both chambers were paced (D, the atrium at a constant rate and the ventricle at a fixed interval--the AV interval--thereafter), neither chamber was sensed (O), and the response mode was neither triggered nor inhibited. In another example, the VVI mode is ventricular inhibited--the ventricle is paced, but only if a naturally occurring R wave does not occur within a set time interval (the occurrence or lack of occurrence being determined by ventricular sensing), and otherwise the pacing pulse is inhibited. Yet another example is the DDD mode, in which both chambers are paced as well as sensed, atrial pacing is inhibited when either atrial activity or ventricular activity is sensed, and ventricular pacing is triggered when atrial activity is sensed. The addition of R as a fourth letter of the pacemaker code, e.g., VVI-R or DDD-R, is indicative of a rate-adaptive capability added to the basic mode.
Selection of the appropriate pacemaker to be implanted is made after the patient has been evaluated, the disorder (e.g., dysrhythmia) diagnosed, and a determination made by the cardiologist that pacing with an implanted pacemaker will provide effective therapy to treat and alleviate the dysrhythmia. As with any therapy, consideration of side effects and contraindications is vital. The techniques of selecting the respective proper pacing modes for treating particular dysrhythmias have been presented in algorithmic form; for example, by M. Schaldach in 1992 (see Electrotherapy of the heart, Springer-Verlag, Berlin (1992)).
For many patients, the progressive nature of cardiac disease necessitates upgrading the patient's implanted pacemaker to accommodate a change in the patient's condition. Typically, this is done by surgically removing what has become a less-effective or ineffective device and implanting a new pacemaker which can provide effective treatment for the existing dysrhythmia. In U.S. Pat. No. 5,609,613, issued Mar. 11, 1997 ("the '613 patent") to the same assignee as that of the present application, improvements are disclosed in artificial pacing for various conditions of patient rest, exercise/activity, and atrial dysrhythmia, and with the advantage of automatic mode switching between dual-chamber and single-chamber modes. Introduction of the fully automatic DDD pacemaker had enabled several clinical aims to be achieved, including adapting ventricular pacing rate to depend on rate of the sensed intrinsic atrial signal, and AV synchrony, but it was subsequently found that some 50% of the DDD pacemaker implant patients were experiencing atrial sensing problems or atrial instability, with underlying atrial rhythm disorders. For those patients, it became necessary to switch from the DDD mode to the simpler VVI mode, by pacemaker replacement. Subsequent improvements in sense amplifiers, electrodes for atrial leads, and timing cycles (e.g., dynamic AV delay and refractory periods) gave DDD pacemakers the capability to provide effective therapy to some of these patients, but a considerable percentage with the dual chamber implants still suffered from inadequate atrial rates. It is estimated that one-third of all patients with sick sinus syndrome (characterized by sinoatrial (SA) arrest or SA exit block) have overly high atrial rates accompanying atrial fibrillation, atrial flutter, or sinus tachycardias including atrial reentry tachycardias and ectopic tachycardiac events (which develop from a focus other than the SA node), as well as slow heart rates.
Proposals have been advanced to discriminate between physiologic and pathologic atrial rates in dual-chamber pacemaker-implant patients for use in switching the pacing mode from an atrial-sensing, ventricular-tracking DDD or VDD mode to a pure ventricular stimulation mode such as VVI or VVI-R. In VVI mode, since pacing and sensing functions take place in the ventricle only, an absence of sensed depolarizations within a set period results in pacing at a programmed nominal rate, and if spontaneous depolarizations are sensed at a faster rate, the pacing pulse is inhibited. But if the atrial detection rate for mode switching is set too low, physiologically-increased atrial rates attributable to physical exercise can cause reversion to an inappropriately low rate; and if the inappropriately high atrial rate detection criterion is set too high, atrial rates occurring with slow atrial flutter, sinus tachycardia, or ectopic beats may be lower and result in overly high ventricular rates despite a resting patient.
To prevent a DDD mode response to an inappropriate high atrial rate by pacing the ventricle at an equally fast rate, the pacemaker is usually programmed with a maximum pacing rate (MPR, or upper rate limit) to produce an abrupt 2:1 AV block. Pacing at the MPR would be undesirable when the patient is inactive because of a tendency to develop symptoms of cardiac insufficiency, lung congestion, shortness of breath, and angina pectoris, under conditions of high myocardial oxygen consumption, particularly if the patient suffers from coronary stenosis. Moreover, the abrupt 2:1 block can cause an undesirable abrupt change in cardiac output and blood pressure, even where high atrial rate is appropriate under conditions of patient exercise. Total atrial refractory period (TARP, which is the sum of the AV interval and the post-ventricular atrial refractory period or PVARP) allows an atrial rate to be reached where sensing of atrial beats cannot occur because they fall in the atrial refractory period.
Rate-adaptive pacing techniques are used to monitor artificial pacing rate and intrinsic heart rate, as well as for controlling the pacing rate to meet the patient's metabolic needs. Proposed sensing for this type of pacing has included central venous blood temperature, QT-interval detection, minute ventilation, intracardiac impedance, and motion detection. For example, the RELAY.TM. dual-chamber, multiprogrammable, accelerometer-based rate-adaptive cardiac pulse generator manufactured by Sulzer Intermedics Inc. (Angleton, Tex.) not only varies the pacing rate according to the patient's level of activity (or lack of activity, i.e., resting) and body position but also monitors the adequacy of triggered pacing of the atrium. (RELAY is a trademark of Sulzer Intermedics Inc.).
In the RELAY.TM. pacing system the MPR is supplemented by a slower interim rate which is greater than the lower rate limit of the device--a ventricular tracking limit (VTL)--to which the pacing rate moves from its base rate conditioned on a high atrial sensed rate without patient exercise. This conditional VTL (CVTL) may be programmed "on" (i.e., as an operating condition of the device) to undergo a controlled jump to the interim rate when a high intrinsic rate is sensed in the atrium without confirmation of patient exercise from the accelerometer. CVTL is overridden when MPR is programmed "on" and the rate calculated from patient exercise exceeds the programmed base pacing rate by a preselected amount--20 bpm, for example. At this accelerometer-based rate threshold, the pacemaker pulse generator restores 1:1 AV synchrony up to the MPR.
The '613 patent discloses a rate-adaptive, dual chamber pacemaker in which the VTL is a dynamic rate, and the ventricular pacing rate is controlled through several different rate zones, based on a combination of (i) dynamic adjustment of VTL according to the accelerometer-based activity signal, and (ii) automatic mode switching from a dual-chamber to a single-chamber mode with reversion to the dual-chamber mode based on an atrial cut-off rate and a programmable rate criterion. Among other things, a mode switch rate (MSR) is designated--above the MPR--that represents an atrial rate unlikely to be exceeded by even a healthy person with a normal cardiovascular system. In one embodiment of that invention, mode switching from DDD-R to VVI-R takes place automatically when the sensed atrial rate exceeds the MSR for a programmed number of consecutive cardiac cycles ranging, for example, from one to seven; and reversionary mode switching reversion back to dual-chamber operation occurs automatically when the sensed atrial rate falls below the MSR for one cycle.
While automatic mode switching is desirable in cases which are amenable to that function to avoid a need for physician reprogramming, there are instances in which progression of cardiac disease mandates a more permanent change in device functionality, or additional features not previously required for control of the patient's dysrhythmia. In the '707 application, an implantable defibrillator having capabilities of artificial pacing, cardioversion and defibrillation is made to be upgradable non-invasively by programming and re-programming the device in a secure manner each time the patient's condition undergoes a significant change, to provide the minimum functional capabilities required to treat the patient's current dysrhythmia, thereby enabling the patient or third party insurer to avoid costs of device features, which, although available in the device, are not presently being used, and of related implant surgical procedures.
It is a principal aim of the present invention to provide an implantable cardiac pacemaker which may be programmed in a secure manner to provide a non-invasive change in basic operating modes when the physician's diagnosis indicates a need for such change to treat a then-current condition. Changes contemplated by the present invention to be available by such programming of the pacemaker include changes from single to dual chamber operation, and addition of features or functions such as anti-bradycardia pacing therapy, anti-tachycardia pacing therapy, and rate-adaptive pacing therapy, and extended memory to record and store intrinsic physiological signals of the patient and their trends over time, such as heart rate and ECG morphology, respiration and other indicators from which to diagnose congestive heart failure and of a special condition of the patient.
Another aim of the invention is to provide an implantable pacemaker which has various therapeutic features and capabilities, some of which may be activated for initial treatment of a patient's pacing problem and others of which can be rendered inactive or disabled for the time being and subsequently selectively made available from time to time only if and when prescribed for treatment of an advanced stage of progressive cardiovascular disease, without a requirement of surgical removal and replacement of the implanted device. Initially, the cost to the patient for the device can be relatively low, limited to that level which is appropriate for the limited features of the device which have been activated. As additional features are activated when needed in the course of treatment of an advancing disease, additional charges may be imposed to allow recovery of costs of development, manufacture, distribution and marketing associated with those features.